1. Field of the Invention
The present invention relates to a method of direct oxidative conversion of xcex2-carotene and xcex2-carotene derivatives such as 3,3xe2x80x2-dihydroxy-xcex2-carotene (zeaxanthin) to 4,4xe2x80x2diketo-xcex2-carotene and 3,3xe2x80x2-dihydroxy-4,4xe2x80x2-diketo-xcex2-carotene (astaxanthin), respectively. In this manner, canthaxanthin, which is a commercial poultry food additive, and astaxanthin, which is a commercial fish food additive, can be prepared from inexpensive naturally derived starting materials.
2. Description of the Related Art
Commercially, astaxanthin is an economically important natural carotenoid which is extensively used in aquaculture to impart natural color in certain species of fish, specifically salmon and trout, as these fish do not have access to these natural pigmentation sources. Likewise, canthaxanthin is a reddish natural pigment which is used in the poultry industry to impart color to the egg yolk and skin. Both compounds are prepared synthetically.
Astaxanthin can be prepared by total synthesis or by algae culture and bacterial fermentation (See, for example, U.S. Pat. Nos. 6,022,701, 6,015,684, 5,972,642 and 5,935,808). The manufacture by total synthesis is costly, requiring expensive raw materials and reagents and is very laborious. The synthesis produces a complete mixture of isomers that are normally not found in nature. The culturing of algae and the fermentation of bacteria produce only low yields of product and mixtures including other carotenoids which make them not suitable for food additives. Likewise, these methods are very costly.
We (patent application No. 09/813,685) have previously described the conversion of zeaxanthin to astaxanthin but the present method provides a cleaner reaction and higher yields. The synthetic conversion of xcex2-carotene to canthaxanthin has been previously described (U.S. Pat. No. 4,212,827). However, this procedure employs expensive iodide reagents or molecular bromine in order to carry out the oxidation. The present procedure employs hypobromide instead of molecular bromine and does not need the 5 day reaction indicated in the above-cited patent. Masuda et al. (Masuda et al. J. (1994) Org. Chem. 59: 5550-5555) reported that hypobromous acid (BrOH) can be generated from NaBrO3 and NaHSO3. Sakaguchi et al. (Sakaguchi et al. (1997) Bull. Chem. Soc. Jpn. 70: 2561-2566) reported that the above reagent can be used in oxidation of various diols to form ketones. However, the use of hypobromide in the oxidation of complex biomolecules such as xcex2-carotene and zeaxanthin to form commercially important pigments has not been reported. The presently disclosed method provides a distinct advantage over previously described methods in that the reaction time is significantly reduced.
In one embodiment of the invention a method of preparing 4,4xe2x80x2-diketo derivatives from xcex2-carotene or xcex2-carotene derivatives is described which includes contacting the xcex2-carotene or xcex2-carotene derivatives in an organic solvent with a solution of an oxidizing agent, wherein the oxidizing agent is formed by mixing acidified aqueous solutions of the sodium or potassium salts of sulfite, hydrogen sulfite, or bisulfite solutions and sodium or potassium bromate solutions in water to produce 4,4xe2x80x2-diketo derivative products. In one embodiment, the xcex2-carotene derivative is zeaxanthin and the product is astaxanthin. In an alternate embodiment, xcex2-carotene is employed and the product is canthaxanthin.
In one embodiment of the described method, the oxidizing agent is formed by addition of an acidified solution selected from the group including sodium or potassium sulfite, sodium or potassium hydrogen sulfite, and sodium or potassium bisulfite to a biphasic media consisting of xcex2-carotene or xcex2-carotene derivatives in an organic solvent and either sodium bromate or potassium bromate in an aqueous solvent. In a preferred embodiment, the oxidizing agent is formed by mixing aqueous solutions selected from the group including acidified sodium or potassium sulfite, acidified sodium or potassium hydrogen sulfite, and acidified sodium or potassium bisulfite with aqueous solutions of either sodium bromate or potassium bromate and adding the resulting solution to an organic solvent solution of xcex2-carotene or xcex2-carotene derivatives. In a most preferred embodiment, the aqueous solution is acidic. In a preferred embodiment, the xcex2-carotene derivative is zeaxanthin and the product is astaxanthin. In an alternate preferred embodiment, xcex2-carotene is employed and the product is canthaxanthin.
In a preferred embodiment of the described method, the organic solvent is chloroform or methylene chloride.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.
While the described embodiment represents the preferred embodiment of the present invention, it is to be understood that modifications will occur to those skilled in the art without departing from the spirit of the invention. The scope of the invention is therefore to be determined solely by the appended claims.
The present invention relates to a method for the oxidative conversion of xcex2-carotene and xcex2-carotene derivatives to the corresponding 4,4xe2x80x2-diketo-xcex2-carotene derivatives. Compounds of economic importance such as canthaxanthin and astaxanthin can be formed readily from xcex2-carotene and zeaxanthin by the disclosed method. The method includes contacting zeaxanthin or xcex2-carotene in an organic solvent with an oxidizing agent formed by the addition of salts of sulfite, hydrogen sulfite or bisulfite to an aqueous solution of a bromate salt. In a preferred embodiment, the salt is a sodium or potassium salt.
In one embodiment, the reaction solvent is a halogen containing solvent and the pH of the solution is acidic. The bromate and sulfite can be mixed either prior to the reaction or added later during the reaction itself. In a preferred embodiment, zeaxanthin or xcex2-carotene is slurried or dissolved in chloroform and an aqueous solution containing sodium or potassium bromate. An aqueous solution of sodium hydrogen sulfite is added slowly to effect the reaction. In another preferred embodiment, an aqueous solution of the oxidant is prepared by mixing sodium bromate and sodium hydrogen sulfite and this solution is added to the chloroform solution or slurry of zeaxanthin or xcex2-carotene to effect reaction.
The present invention also may have some utility in the formation of 4,4xe2x80x2-keto groups of other carotenoid compounds. Such compounds include but are not limited to echinenone, astacene, phoenicopterone, 3-hydroxycanthaxanthin, and 3,3xe2x80x2-dihydroxyechinenone.
The present invention is directed toward a method of treating xcex2-carotene and xcex2-carotene derivatives with an oxidizing agent. Non-limiting examples of performing allylic oxidations or agents that may be employed in oxidation reactions have been reviewed in Barry Trost and Ian Fleming, eds. xe2x80x9cComprehensive Organic Synthesisxe2x80x9d, Volume 7, Pergamon Press, New York, 1991, Pages 83-117, and Richard C. Larock xe2x80x9cComprehensive Organic Transformationsxe2x80x9d, Wiley-VCH, New York, 1999, pages 1207-1209, which are incorporated herein in their entirety by reference. However, most of these conditions produce substantial or complete decomposition of the sensitive xcex2-carotene structure. In contrast, in the preferred embodiment of this invention the double allylic oxidation of this sensitive structure can be carried out in good yield. Preferably, zeaxanthin with or without a blocking group on the alcohol moiety can be oxidized directly to astaxanthin or astaxanthin derivatives by the activity of an oxidant derived from sodium or potassium bromate and sodium hydrogen sulfite or sodium metabisulfite under acidic conditions.
In one embodiment of the invention, the reaction is carried out in a two-phase system utilizing a chlorinated solvent in the second phase along with the aqueous phase of sodium or potassium bromate to which is added a solution of sodium hydrogen sulfite. In a preferred embodiment, the organic solvent is chloroform and the aqueous solution is pH=1-3. Also, preferably, an oxidizing solution can be prepared and stored by mixing together sodium or potassium bromate and either an acidified solution of sodium sulfite or solutions of sodium hydrogen sulfite or sodium metabisulfite. Preferably the pH of the resulting media is between pH=1-3.
In an alternative embodiment, xcex2-carotene instead of zeaxanthin can be employed as the starting material and the reaction will produce canthaxanthin.
In one embodiment of the invention, the ratio of oxidizing agent relative to zeaxanthin or xcex2-carotene can vary between a catalytic amount to 2 molar equivalents. The term xe2x80x9ccatalytic amountxe2x80x9d refers to an amount of the oxidizing agent that is less than the stoichiometric quantity of the zeaxanthin or xcex2-carotene used in the reaction. The term xe2x80x9cstoichiometricxe2x80x9d refers to the use of an equivalent mole ratio or amount of a reagent relative to a selected substrate, molecule or compound in the reaction.
In one embodiment of the invention, the reaction is carried out in a two phase system with an inert organic solvent layer and an aqueous layer. The organic layer can be any inert solvent, but preferably, chloroform or methylene chloride are used.
In one embodiment of the invention, the ratio of zeaxanthin or xcex2-carotene to organic solvent varies from 1:10 to 1:500 depending upon the reaction conditions. Preferably, the ratio of zeaxanthin or xcex2-carotene to the organic solvent is from about 1:10 to about 1:200.
In one embodiment, the reaction is conducted under acidic conditions. In a preferred embodiment, the pH of the reaction media can be from pH=0 to pH=6. In a most preferred embodiment, the pH is between 1-3. Any non-reactive acid may be employed to adjust acidity. In a preferred embodiment, the acidity is increased by the addition of sulfuric acid.
In one embodiment of the invention, the temperature of the reaction may vary from 0xc2x0 C. to 50xc2x0 C. In a preferred embodiment, the reaction is carried out between 10xc2x0 C. and 40xc2x0 C.